VW boss Martin Winterkorn said he was "endlessly sorry" over the scandal, which has prompted questions about his future.

Air quality campaigners have called for the Government to launch an inquiry into whether cars on Britain's roads also broke the rules, but the Society of Motor Manufacturers and Traders (SMMT) insisted that cars sold in the UK "must comply with strict European laws".

The Environmental Protection Agency (EPA) in the US said cars had been fitted with sophisticated software to switch engines to a cleaner mode when they are undergoing official emissions testing.

This is a type of software known as a "defeat device". Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.

Volkswagen now faces the cost of recalling millions of vehicles as well as a fine of up to $18bn (£11.6bn) in the US.

Authorities across the world have launched further probes and Transport Secretary Patrick McLoughlin has written to the European Commission to call for an investigation.

He wrote: "Like the UK, I am sure the commission is keen to reassure drivers and use this moment to demonstrate that the European engine testing regime is robust.

"I am writing to seek your assurances that the European Commission will investigate this matter thoroughly and take appropriate action to avoid a reoccurrence."

German Chancellor Angela Merkel demanded "full transparency" from the company, adding she hopes "the facts will be put on the table as quickly as possible".

An inquiry is under way in South Korea covering several models. There are calls by France for a Europe-wide investigation into VW as well as French car-makers' practices. The European Commission has contacted VW and the US authorities over the findings.

VW said it was "working at full speed to clarify irregularities concerning a particular software used in diesel engines" and has found "discrepancies... involving some 11 million vehicles worldwide".

The manufacturer said the issue related to type EA 189 engines. It said: "A noticeable deviation between bench test results and actual road use was established solely for this type of engine."

What now?

According to the AA, close to a million vehicles are called back to dealers every year for a safety check and/or rectification work under the official vehicle safety recall scheme.

Where a safety defect has been identified and registered, the manufacturer or importer will liaise closely with the authorities and write to registered keepers of affected vehicles explaining clearly what the defect is.

They will also tell you what can happen if it's not attended to, what repairs are required and what action you should take. You must ensure that your vehicle is maintained in a safe and roadworthy condition. Ignoring a safety recall could also affect an insurance claim. The recall work should be undertaken free of charge.